Contributions of morphospace and multivariate analyses to interpretations of phenotype patterns and trends in Mexican Idoceras

摘要

Morphospace analysis and multivariate statistics are conducted for the first time on Mexican Idoceras, a Kimmeridgian ammonite genus known for more than one hundred years. The database investigated contains 108 specimens belonging to old collections and material collected under precise biostratigraphic control from 1985 onwards. The main objectives are to evaluate phenotype dynamics, in terms of morphologic patterns through the numerical evaluation and then interpretation of the proposed Standardized Morphotypes. The phenotype variation was analyzed through Raup's theoretical morphospace and compared with the observations obtained from representatives of the genus. The subsequent comparative analysis using Multivariate statistics through Discriminant Analysis, Principal Component Analysis, and Cluster Analysis highlights precise relationships between the selected Standardized Morphotypes and two assemblages of Idoceras previously established with recognized stratigraphic value. These analyses reveal persistent relationships among classic and Raupian variates, which points to covariation of shell features within an evolutionary line, here the case of Mexican Idoceras, while differences related to shell-size are interpreted as resulting from ontogenetic changes in shell morphology derived from the influence of differential age and/or growth rate in phenotype expression. An attempt is made to interpreting the morphological trends represented by the Standardized Morphotypes in terms of shell hydrodynamics and behavior of ammonites under changing paleoenvironmental conditions.